The present invention relates to liquid chillers of the type which provide chilled water for industrial process and/or comfort conditioning applications. More particularly, the present invention relates to a screw compressor-based water chiller and the control thereof. With still more particularity, the present invention relates to start-up procedures for screw compressor-based water chiller systems, detection of a so-called inverted start conditions in such systems and control of such chillers to address the inverted start circumstance.
At and during the start-up of a refrigeration chiller, the majority of the chiller's refrigerant charge will normally be found in the shell of the system evaporator. This is because refrigerant, by its nature, tends to migrate to and settle in the coldest part of a chiller system when a chiller is not in operation and because the system evaporator will be the coldest location in the chiller for some period of time subsequent to its shutdown and, normally, when it next starts up. Also, pressure across a chiller system will typically have equalized during a shutdown period due to leakage paths that come to exist across the system only after it shuts down.
During "normal" start-up of a chiller, the system expansion valve, which meters refrigerant from the high pressure side ("high-side") to the low pressure side ("low-side") of the chiller system, is typically prepositioned to a nominal, more closed setting. Positioning of the expansion valve to the more closed setting occurs under the presumption, for the reasons noted above, that there is a sufficient amount of refrigerant in the system evaporator at chiller start-up to supply the system compressor until steady state operation is achieved.
Prepositioning of the expansion valve to such a relatively more closed position is done to allow a pressure differential to build up quickly between the high and low pressure sides of the chiller system, the boundaries of which are the system's expansion valve and compressor. The relatively quick buildup of such differential pressure at chiller start-up is necessary and critical in such systems because it is that pressure differential which is used to drive oil from its storage location in the chiller to the surfaces and bearings in the chiller that require a supply of oil in order to function. To further ensure a safe start for the chiller under such "normal" start-up conditions, a time delay may be built into the chiller's control logic only after which will the chiller be permitted to load.
In view of the above regarding refrigerant charge location under normal start-up circumstances, if the sensed evaporator leaving water temperature (the temperature at which the water leaves the evaporator after having passed through the tube bundle therein) is lower than the sensed condensing water temperature, current chiller systems presume that the majority of the system's refrigerant charge is in the system evaporator rather than the condenser. This is because, once again, refrigerant, by its nature, will tend to migrate to and settle in the coldest part of a chiller system when the system is not in operation. Colder evaporator water temperature is thought to confirm the presumption. Under such circumstances, "normal" chiller start-up logic will be used to bring the chiller on line with the expansion valve being positioned to a relatively closed down position.
The circumstance where a majority of a chiller system's refrigerant charge is in the system condenser rather than the system evaporator at start-up is referred to as an inverted start condition. In current chiller systems, the fact that sensed evaporator leaving water temperature is higher rather than lower than sensed condenser water temperature is presumed to indicate that the majority of the system's refrigerant charge is in the condenser rather than the evaporator and that an inverted start condition exists.
Inverted start conditions require that a unique control sequence be employed in starting the chiller due to the presumed unavailability of a sufficient quantity of refrigerant in the system evaporator to adequately feed the system compressor in the face of what would, under normal start-up conditions, be a relatively closed-down expansion valve. Absent an adequate supply of refrigerant in the system evaporator at start-up, buildup of an adequate pressure differential between the high and low-sides of the chiller system may not occur. That, in turn, jeopardizes the supply of lubricant to the compressor at start-up and the chiller may be subject to repeated failed starts or shutdowns, under a low oil pressure diagnostic, before conditions internal of the chiller "normalize" and a successful and sustained start can be achieved.
Currently, when the existence of an inverted start condition is suggested by virtue of the fact that condensing water temperature is sensed to be lower than evaporator water temperature, "inverted start-up logic" is used to start the chiller. That logic typically includes a pre-start step of opening the system expansion valve to a relatively more wide open position than would be found under "normal" start conditions. By so positioning the expansion valve, quick relocation of the refrigerant charge from the system condenser to the system evaporator is sought to be achieved. However, by virtue of the fact that the system expansion valve is so-positioned and constitutes a boundary between the high and low pressure sides of the chiller system, a relatively open flow path between the high and low-sides of the chiller system is caused to exist which is, in its own fashion, detrimental to the development of a pressure differential between the high and low pressure sides of the chiller. Further, in chiller systems where compressor loading is delayed during "normal" start-ups as an added measure of compressor/chiller protection, such delayed loading is often dispensed with under inverted start conditions due to the need to drive refrigerant out of the condenser and into the evaporator. The use of inverted start logic is therefore to be avoided if possible for the reason that a measure of safety is lost in terms of protecting the compressor as it starts up.
Still further, the fact that condenser water temperature is lower than evaporator water temperature at start-up, while normally a good indicator of the existence of inverted start conditions, is not a foolproof indicator. For instance, when a refrigeration chiller is used in conjunction with condensing water supplied from a cooling tower, the start-up of cooling tower pumps can cause water to flow to the chiller's condenser which is initially colder than evaporator leaving water temperature. Under that circumstance, the fact that the condensing water temperature is colder than evaporator leaving water temperature is not a reliable indicator of an insufficient refrigerant charge in the system evaporator to sustain chiller start-up (even though that may, in fact, be the case). Therefore, false indications of the existence of inverted start conditions can occur and inverted start-up logic is sometimes used when it is not called for. Use of inverted start-up logic when it is not, in fact, called for can cause extended refrigerant floodback to the compressor and no or low refrigerant superheat to be achieved, all to the detriment of chiller operation.
In a similar manner, there are certain circumstances where the use of inverted start logic is, in fact, called for but comparative evaporator and condenser water temperatures do not suggest the existence of the condition. As a result, "normal" start-up logic is sometimes used when inverted start logic is actually called for.
In both of these cases of erroneous indication, chiller shutdowns and failed starts often result, to the detriment of the industrial process or building comfort application in which the chiller is used. The need therefore exists to better determine the existence of inverted start conditions in refrigeration chillers and to better address those conditions when they do exist so that unnecessary failed starts and chiller system shutdowns are reduced or eliminated.